Bhattacharya, A K

Jaman, A I

Nandi, R N

IACS Institutional Repository

IxIim J. Pkys. 788(1), 75-80 (2004)a: UPInfrared-radiofrequency double resonance spectrometer: an unique machine to probe excited vibrational statesR N Nandi*, A K Bhattacharya and A I Jaman*Saha Institute of Nuclear Physics, Sector-l,jBlock-AF, Bidhannagar, Kolkata-700 064, IndiaLli-mail : jamanpcmp.saha.ernci.in Received 28 July 2003, jficcepted 12 November 2003Abstract : The construction of an Infrared-Radiofrequency Double Resonance (IR-RF DR) spectrometer to study ‘pure’ nuclear quadrupole hyperfine spectra in the ground and excited vibrational state of a molecule in the gas phase is presented An indigenously built COi laser was used as the Infrared radiation source. The spectrometer has been used to observe ‘pure* nuclear quadrupole hy^ xiiTine spectra of methyl iodide (CHd), a symmetric top molecule with a single quadrupole nucleus.Keywords : Molecular spectroscopy, quadrupole hyperfine spectra, excited stale.PACS Nos. : 33.20.Bx. 33.15.Pw1. IntroductionIn a particular vibrational state, microwave rotational spec­tral lines arise from transitions between two quantized rotational energy levels which obey Boltzmann’s distribu­tion law at thermal equilibrium and get splitied into differ­ent quadrupole hyperfine components in presence of a quadrupole nucleus (nuclear spin >\Zi) in the molecule. The separation between different quadrupole sublevels, in gen­eral, falls in the radio frequency region of the electromag­netic spectrum.Microwave rotational spectra and ‘pure’ nuclear qua­drupole hyperfine spectra in the excited vibrational state of a molecule in the gas phase are difficult to observe due to the fact that the excited vibrational states are not sufficiently populated at ambient temperature because of the Boltzmann distribution law. However, the problem of insufficient population can be overcome by ‘pumping’ molecules from the ground to the desired excited vibra­tional state with a suitable laser radiation. Once the ‘pump­ing’ is achieved, the molecular sample can be irradiated with an appre^ate  microwave or radio frequency radiation depending upon the nature of interest and the spectro­scopic studies can be conducted easily in the desired excited vibrational state. This technique is known as Infra- red-Microwave (IR-MW) or Infrared-Radio Frequency Double Resonance (IR-RF DR) technique and has been pioneered and extensively used by Oka [1-4] for various studies of molecular properties in the excited vibrational states.The intensity of radio frequency absorption spectrum is very small because of the low energy of radio frequency photons and due to the small population difference be­tween the two levels. As a result, conventional radio frequency spectroscopic technique could not become a popular spectroscopic tool. In an IR-RF DR experiment, these difficulties have been overcome by detecting infra­red laser radiation having larger photon energy rather than the RF radiation and by producing a non-Boltzmann dis­tribution with an infrared laser ‘pumping*.In the present communication, the design and con­struction of an IR-RF DR spectrometer is presented. The spectrometer has been used to observe ‘pure’ nuclear quadrupole spectrum of gaseous methyl iodide (CH3I) in its ground and excited vibrational state v^ .Corresponding Author 'Deceased © 2004 lACS76 R N Nandi, A K Bhattacharya and A /  Jaman2. Experimental set-upA block diagram of the experimental set-up for Infrarcd- Radiofrequency double resonance spectroscopy is shown in Figure 1. A coaxial rf cell (details shown in Figures 2(a)tion, is chosen by rotating a plane grating. RF radiation obtained from a Frequency Synthesizer (Rohde-Schwarz Model SMG-Bl, 100 kHz-1000 MHz) at a power level of -15 dBm was amplitude modulated at 10 kHz with the helpPUIK)DiffusionandMechofticoiHg-Cd-Te detoctor (Liquid Nitrogen cooled)Zn-ScwindowDios secHon for VQCcuin and sample hondlinqnJl ......Copper  ^ - vvelectrodes^ p^'SeconcQve h ..A nilrrnr >SOKiVreflectingCO2 -Loser Power supply 3a kV 50mACos discharge tubeBeamsplithfrs mixture out 'tn***^ J  |iSpecfnimAnalyzerC02LoserCOa-Losar Power meter■aTUir ItI2+CO2+ N^ )R.F. Power Amplifier 5DWR DoubleBoloncemixerL Sweepable R.P. SourceRotinum electrodeGold ploted K.y Grating^^Z n~Se brewster windowDouble pen lock in 10 kHzchart recorder. .. . . rpsoOutputAmpUfier "^ Ref Squorewove1 —_f Signal GMwrotefFig. 1Figure 1. Block diagram of Infrared-Radiofrequcncy Double Resonance SpectrometerCyOndrlcal copper electrodes Loser btonEtectAideterminalsFig. 2(b)Window holderof oluminiumFigure 2 (,). Details of coaxial RF cell design, (b). Laser window holders for coaxial RF celtCylindrical inner hollowCopper electrode d<amef«wt/2*Outer copper tlectrode dkUMterand 2(b)) containing the gaseous sample under investiga­tion at low pressure is irradiated with an infrared laser radiatiOT pnxiuced with an indigenously built CO  ^ laser. The laser beam after passing through the cell falls on a crystal detector through a 10% transmission curved miiror. The infrared laser line, coincident with a particular vibra­tion-rotation transition frequency of the molecular absorp-of a double balance mixer (HP 10514A) and fed to a power amplifier (ENI 550L). The amplified RF radiation was sub­sequently fed to the absorption cell which was terminated to the ground through a 50 i2 resistor. The sample pres­sure was maintained at around 10-20 mTorr wiA the help of a combination of appropriate mechanical and diffusion pamp. TTie laser output was monitored witii a liquid nitro­Infrared-Radiofrequency double resonance spectrometer: an unique machine etc. 77gen cooled Hg-Cd-Te detector (EG & G Judson, USA). In this experiment, the laser frequency was kept fixed and the RF radiation was scanned. When the RF frequency passes through a molecular absorption, the load characteristics of the gaseous sample changes and a shaip variation of the laser output power is observed as a double resonance signal. The detector output was accordingly modulated at 10 kHz, phase-sensitively detected by a lock-in-ampIifier and recorded on a chart recorder.returned to the ground through a ballast resistor of 50 Ki3^400 W rating. A Matheson (USA) flowmeter was used to maintain the flow of a mixture of CO2, Nj and He gases through the discharge tube. The discharge tube was con­nected to a mechanical pump. An approximate ratio o f 1 ; 1 ; 4 was maintained for CO2, Nj and He through the flowmeter and was varied sometimes to achieve maximum laser power. The -discharge tube was placed inside aNeoprenegasket2.6—►Brewster angleQmi tubeO.D.3l3inmRKtongulor brewster windowFig. 3(b)Figure 3 (•). Details of COj-laser gas discharge tube design, (b). Details of Brewster window holder design.3. CO2 laser construction : a brief descriptionA 1.7 meter long pyrex tube with an inner diameter of 10 mm and outer diameter of 13 mm has been used as the discharge tube (details shown in Figures 3(a) and 3(b)). The outer wall of the discharge tube is surrounded with a glass jacket for water-cooling. The two ends of the dischaiige tube are closed with rectangular Brewster win­dows which hold the gas mixture at desired pressure (7- 10 Torr). The Brewster’s windows are made of ZnSe which permits the transmission of infrared radiation of the CO2 laser as well as the visible radiation of He-Ne laser. The visible radiation is used for the alignment of optical com­ponents. The D.C. discharge in the tube is maintained by applying a high negative voltage (-15 KV) at the cathode end. For this purpose, a high voltage D.C. regulated power (30 kV, 66 mA) procured from Glassman, Japan (Model LT) has been used. A platinum cone was used as the cathode and a tungsten wire as anode. The anode wasFebry-Perot cavity consisting of a curved mirror (10% transmission) and a grating separated by an approximate distance of 2 meters. The cavity length was kept fixed by using three pieces of Invar rods fixed on two triangular shaped metallic blocks. Two kinematic mounts have been used to place the partially transmitting mirror and the grating. The kinematic mount at the grating end can be adjusted for selecting a particular laser line of interest. In order to identify the actual laser line during lasing, a part of the laser radiation coming out of a partially transmitted mirror (10% transmission and 90% reflection) placed after the cell is extracted by a beam splitter and allowed to fall on a CO2 spectrum analyser (Model 16-A, Optical Engi­neering, Lasercraft Inc., USA). From the calibrated display screen of the spectrum analyser the wavelength o f the laser radiation can be obtained. Other part o f the tagpr is fed to a CO2 laser power meter (Scientech, model 37-4(X)2) for measuring the laser power.78 R N Nandi, A K Bhattacharya and A I Jaman4. Nuclear quadrupole hyperTine structure of methyl iodide (CH3I)Methyl iodide is a symmetric top molecule where the rotational state in any particular vibrational state is de­noted by the eigenstate 17 /0  where 7 is the rotational angular momentum and K is its projection along the molecular symmetry axis. In the CH3I molecule, a hyperfine splitting occurs because of the interaction between the iodine nuclear quadrupole moment (nuclear spin = 5/2) and the electric field gradient produced by the electron cloud at the quadrupole nuclear site. Due to this interaction, 7 gels coupled with I to produce the resultant angular momentum F (F — J + I) which can take values from |7— /I to I7 + / | .  The energy level diagram for vibration- rotation transition 166 15s of ik, fundamental band ofCH3I is shown in Figure 4.♦ r ~ 1  31/216 6 t - - - - -  .1 I* . .1 — 14+1 29/(Excited State) 37^ ^' " - J - I  27/210P132) Intmred-— Loser (Coj)Loser tronsHiont A J* AF-1J K A ' ' '15 S ----V . 0(Ground State)RF tronsUion A J a O  A K s O  AF =1—-!n iT J + I -2  31/2 29/2■I J*I-1  33/2. j |«6J-I+1 27/2>J+IJ - I35/225/2Figure 4. Energy level diagram for vibration-rotation transition 16t4-13] of Vt fundamental band of CH3I.The dipole selection rule /MT = 0, and + -♦  -  for rotational transitions in a symmetric top molecule allow transitions with AJ — 0 and AF — 1 between the hyperfine levels of rotational states with if  ^  0, because these states have a double parity. In Figure 4 the quadrupole splitting in the lower and upper states of an infrared transition are schematically shown. Five if  transitions are allowed in each state and are denoted (x, y, S, and E in the ground state and a \ y '. S', and s' in the excited state.Since the hyperfine splitting has a similar pattern in the lower and upper state, the infrared transitions with AF = AJ are close in frequency which spans a frequency range comparable to the Doppler width o f the methyl iodide vibrational transitions. Hence, the laser line can often pump all these transitions and as a result, all the five resonances are observed in both states in a double reso­nance experiment. Conversely, the AF * AJ infrared tran­sitions have a longer spread in frequency and the laser has coincidence with only one or two of them. As a result, not all hyperfine levels are pumped and only a few rf resonances are observed in double resonance experiment. The nuclear quadrupole energy expression [5] for the iodine nucleus on the axis of symmetry isWfJilrJ,F) = eqQ [ 3/f^/7(7 + 1) -1] Y{UF) +where eqQ is the quadrupole coupling constant, Y(I,J,F) is the Casimir function and is the coefficient of second order perturbation contribution on the rotational energy levels and B is the rotational constant.5. Perform ance of the spectrom eterThe performance of the spectrometer described above has been checked with a test sample of methyl iodide. Methyl iodide has a number of normal vibrational frequencies. Its H, vibrational mode coincides with die 10 /an  region of the CO2 laser. The ‘pure’ nuclear quadrupole resonances of gaseous CH,1 have been investigated in detail by Oka [2] using Infrared-radiofrequency double resonance spectros­copy. Therefore, precise values of quadrupole resonance frequencies are available in Ref. [2]. First of all, we looked for the double resonance signal ^/?(15j), vfc (V = 1)<— 0 transition of methyl iodide. Here, the infrared transition connects the 7 = 15, /f  = 5 level of the ground state with the 7 = 16, E  = 6 level of the excited vibrational state vi. The 10P(32) line of the CO2 laser matches this separation. The ‘pure’ quadrupole resonance signal of F  = 31/2 ^  29/ 2 for the excited state V6 at 100.123 MHz [2] was attempted first. The laser was tuned to 10F(32). The sample pressure at the coaxial rf cell was kept around 15 mTorr. The laser power was set at around 1.5 W. Under this condition, the rf frequency was swept ±4 MHz around 100.123 MHz. A strong double resonance signal was observed when the rf generator frequency passes through 100 MHz. The genuinness of the double resonance signal was checked by pumping out the sample gas from the absorption cell while recording a repetitive scan o f the signal. A gradual decrease in signal intensity confirms that this is a genuine double resonance signal. The repetitive scan o f the signalInfrared-Radiofrequency double resonance spectrometer: an unique machine etc. 79is shown in Figure 5. After this double resonance signal was confirmed, many other double resonance signals e.g., F  = 29/2 27/2 {v  = 0) at 114.72 MHz, F  = 31/2 <-33/2region. It is to be mentioned here that we have been able to observe some more double resonance signals using 10P(4) laser line as ‘pump’ radiation which are not reported" 100.123MHj'' 100.123MHi 100.123MHj 100.123MHj 100.123MHzFigure 5. Repetitive scan of 100.123 MHz IR-RF DR signal (F = 31/2 4— 29/2, Ki (i> 5= l)<—0) in pump-on condition.(y  = 0) at 82.92 MHz and F  = 33/2 <- 35/2 (y  = 1 of band) at 73.92 MHz were observed. These signals are shown in Figure 6. The same method of observation as stated earlier was followed. All the signals were verified by pumping out the sample from the absorption cell. As mentioned earlier that in principle, we should be able toUmt fraq.10P(lZ>Prmurt-IOdTorr L a i t r  p o w t P ^ 1 5 W / 1 0 0R.FJLSJLS « n * i t i v i t y * '2 0 n i vPhoit^OTiiR« count • 1 ooc.Figure 6. Other observed double resonance signals.here. The observed ‘pure’ nuclear quadrupole resonance frequencies in the ground and V(, excited state are pre­sented in Table 1. The quadrupole coupling constants eQq of CHjl have been determined to be -1934.136 MHz and -1940.50 MHz for the ground and excited vibrational state respectively 12).Table 1, ‘Pure’ quodrupole resonance frequencies (MHz) of CH.,1 observed in the ground and exited state.Ground staleLaser line ‘^ CHJ lineexited statep O' P’ S '82.92 25.58' 114.72 185.80“ 185.80“ 73.92 21.05' 100.12'not seen, ‘‘unresolved doublet.observe 10 (5 each for ground and V(, excited vibrational state) double resonance signals with 10F(32) laser line. Hence, we looked for the remaining six double resonance signals for the sake of completeness. However, we have been able to observe three more double resonance signalse.g„ at 164.4 MHz (F  = 29/2 <- 27/2; t/ = I of Vfc band), a strong signal at 185.8 MHz (F = 27/2 <-25/2; y = 0 and F  = 35/2 <- 37/2, y = 1 of ^  band coinciding with each Other) and at 211.8 MHz (F = 33/2 <- 35/2; y = 0). The remaining two transitions (F  = 33.'2 <— 35/2; y = 1 of F6 band and F  = 31/2 « - 29/2; y = 0) which arc at 21.05 MHz and 2S.S86 MHz respectively could not be observed very clearly because o f the RF source noise in this frequency6. ConclusionThe details o f the construction o f an Infrared- Radiofrequency Double Resonance spectrometer has been presented. The spectrometer has been successfully tested to observe many double resonance signals for a number of vibration-rotation transitions of the test sample CH3I. The genuineness of the signals were verified by pumping out the sample from the absorption cell which shows gradual decrease in intensity. This spectrometer has proved to be an excellent tool for probing molecular energy levels in the excited vibrational states. It is to be mentioned here that this method o f observing ‘pure’ quadrupole resonance80 R N Nandi, A K Bhattacharya and A I Jamanfrequencies of gaseous samples is applicable to any polar symmetric top molecule with a single quadrupole nucleus. Work is underway to incorporate some new devices to tune the laser over its entire gain-bandwidth curve so that the laser can be brought into resonance with some of the vibration-rotation transitions which are offset by 50 MHz or so.References[ 1 ] T Oka Frontiers in Laser Spectroscopy VoLII ( North Holland : Les Houches) (1975)[21 E Arimondo, P Gloricux and T Oka PhysMev. A17 1375 (1978)[3] F Scappini and T Oka J, Mol Spectrosc. 85 390 (1981)[4] P Gloricux, E Arimondo and T Oka J.Phys.Chem. 87 2133 (1983)[51 W Gordy and R L Cook Microwave Molecular Spectra (New York : John Wiley and Sons) p412, p426 (1984)

Infrared-Radiofrequency Double Resonance Spectrometer : an Unique Machine to Probe Excited Vibrational States

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/7006/1/Infraded-Radiofrequency%20Double_By%20R%20N%20Nandi.pdf

Infrared-Radiofrequency Double Resonance Spectrometer : an Unique Machine to Probe Excited Vibrational States

Abstract

Similar works

Full text

Available Versions

IACS Institutional Repository